Method for attenuating float glass

Abstract

The marginal edge portions of an accelerating ribbon of float glass are engaged by a plurality of opposed pairs of attenuating devices, each pair exerting substantially equal forces. The pairs of attenuating devices are positioned longitudinally adjacent one another such that the ratio of the ribbon width intermediate adjacent pairs to the average of the ribbon widths at each pair is greater than 0.95. The attenuating devices may be edge rolls each rotating with a peripheral velocity a predetermined amount greater than the linear velocity of the ribbon immediately upstream.

I claim: 
     
       1. In a method of attenuating a ribbon of glass by a plurality of opposed pairs of attenuating devices spaced longitudinally within an attenuating zone of a flat glass forming chamber, each of said attenuating devices engaging adjacent marginal edge portions of the ribbon as the ribbon advances downstream through the attenuating zone upon a pool of molten metal toward a discharge end of said chamber, the ribbon having internal forces which tend to maintain the ribbon in an equilibrium condition and resist attempts to attenuate the ribbon from its equilibrium condition, wherein said attenuating devices are spaced and actuated to exert longitudinal and lateral attenuation forces on the ribbon which act against the internal forces of the ribbon to generate non-uniform longitudinal and lateral stresses in the ribbon within the attenuating zone, which non-uniform longitudinal and lateral stresses detrimentally affect the optical quality of the final ribbon, the improvement comprising practicing the steps of: longitudinally spacing said plurality of opposed pairs of attenuating devices in such proximity relative to one another in said attenuating zone such that the width of the ribbon is maintained between adjacent pairs of attenuating devices at least about 95 percent of the average ribbon width measured at said adjacent opposed pairs of attenuating devices; and   rotating said plurality of opposed pairs of attenuating devices with progressively increasing rotational speeds relative to the preceding opposed pair to accelerate the engaged portions of said ribbon with each of said plurality of opposed pairs of attenuating devices, wherein each of said plurality of opposed pairs of attenuating devices is rotated with a rotational speed which has a corresponding longitudinal component of velocity which is a predetermined amount ΔV greater than the longitudinal velocity of the ribbon as measured immediately upstream from each of said opposed pairs wherein, the predetermined amount ΔV is maintained substantially equal for each of said plurality of opposed pairs of attenuating devices to minimize non-uniformities in the longitudinal stresses in the ribbon in the attenuating zone.   
     
     
       2. The method as set forth in claim 1, further comprising the step of: adjusting the rotational speed in combination with the angular orientation of each of said opposed pairs of attenuating devices to maintain said substantially equal ΔV while altering the lateral component of velocity of said opposed pairs sufficient to minimize non-uniformities in the lateral stresses of the ribbon.   
     
     
       3. The method as set forth in claim 2, wherein the longitudinal component of velocity of each attenuating device is between about 5% and 10% greater than the longitudinal velocity of the ribbon measured immediately upstream of each of said attenuating devices. 
     
     
       4. The method as set forth in claim 3 wherein the attenuating devices comprise edge rolls having incrementally increasing circumferential dimensions from the upstream pair to the downstream pair and wherein said rotating step includes: rotating said edge rolls at substantially equal angular velocities.   
     
     
       5. The method as set forth in claim 3 wherein the attenuating devices comprise edge rolls having substantially equal circumferential dimensions and wherein said rotating step includes: rotating said edge rolls at incrementally increasing angular velocities from the upstream pair toward the downstream pair.   
     
     
       6. The method as set forth in claim 1 or 3 wherein each of said attenuating device includes a single elongated fluid cooled member having an interior end in said chamber and an exterior end outside said chamber, and wherein the ribbon has an undesirable transverse temperature profile which is aggravated by the extraction of heat from said chamber through said fluid cooled member, further comprising the step of: mounting a plurality of edge rolls adjacent the interior end of the single elongated fluid cooled member spaced from one another longitudinally relative to a marginal edge portion of the ribbon prior to practicing said longitudinally spacing and rotating steps, wherein said mounting step is practiced in a manner to reduce the amount of heat extracted from the marginal edge portions of said ribbon through said single fluid cooled member to a level less than that which would be extracted by a plurality of similar elongated fluid cooled members each supporting one of said plurality of mounted edge rolls, to reduce the effect of practicing said longitudinal spacing and rotating steps upon said transverse temperature profile.